Many mechanical devices which utilize a lubricated rotating shaft as a component require seals on those shafts to either contain a pressurized fluid, e.g., for a shaft driving a pump, or to isolate one component from contamination, or both.
In the case of downhole rotating equipment, such as an electrical submersible pump (ESP), or a geared centrifugal pump (GCP), the rotating shafts require seals, principally to protect the fluids inside sensitive components, such as the electric motor (ESP) or the transmission (GCP), from contamination such as from production fluids.
The seals in these two applications are not called upon to withstand significant a pressure differential, as they are typically equipped with pressure compensators that keep both sides of the seal at near equal pressures. This is important in downhole equipment applications, as the devices are designed to operate essentially maintenance and service free, while at full power, and for years, so leakage across the seals must be kept to a minimum. If, as and when maintenance or repair is called for, the entire string must be pulled adding an inordinate loss of time and consequent expense.